Basically, when you are cloning, you are trying to get adventitious roots to develope from stem tissue. Adventitious roots are roots that develop from non root tissue per se, but there is an exception. They can naturally form, or be the response of a wound or infectious disease. They can be formed on older mature wounded roots and the wound's associated callus tissue. They may develop on excised plant parts (cuts) or in tissue cultures. Adventitious roots can form on many parts of the plant, including stem nodes AND internodes. Some species even develop adventitious roots from leaves as does the common begonia. They usually arise from Parenchyma cells found closely associated with the plant's vascular tissue. In young stems, the adventitious roots arise from interfascicular parenchyma and in older stems with secondary growth, from the vascular ray near the cambium. The cambium is the tissue that gives rise to but also seperates the the secondary xylem and phloem in woody plants. When adventitious roots are formed in cuttings they MAY originate in the callus tissue that is often formed at the base of cuttings.
At the tip of a root or shoot tip there is a small cluster of cells called meristem cells and make up what we call the primordia. Meristem cells are basic plant cells that have yet to be differentiated into into any of the specialized cells that make up a plant. Most tissue culture is based on collecting these meristem cells.Xylem, phloem, and parenchyma cells are just some of the specialized cells a meristem cell could become. Parenchyma cells are the main "filler" cells of most plant parts or organs. They are found containing chloroplasts in leaves, and between the vascular bundles of leaves, stems and roots. They give the bulk in younger plant tissues. They carry on a variety of functions depending on their location within the plant. However, what makes them unique is that they are capable of changing their activities as the plant's needs change. This is a result of them maintaining a complete protoplast during their differentiation. A parenchyma cell with a normal nucleate protoplast can resume meristematic activity. The phenomena of wound healing, regeneration, formation of adventitious roots and shoots, and union of grafts are all made possible through a resumption of meristematic activity by parenchyma cells.
Therefore, the adventitious root tip (primordia) is initiated by divisions of parenchyma cells whether callus cells or other parenchyma cells. If from within the stem, it initiates from the parenchyma between the xylem and phloem. Auxins are associated with stimilating undifferentiated cells into forming root structures. Before the adventitious root emerges from the stem or root it differentiates an apical meristem, a rootcap, and the beginning of the vascular cylinder (to become the xylem and phloem), and the cortex. When the xylem and phloem (vascular structures) are differentiated in the adventitious root, the callus or other parenchyma cells that lay between it and the plant's existing xylem and phloem differentiate to become the same, thus connecting the vascular structures. Here's a side view of a cross section:
This image should help figure out some of the structures mentioned above, collenchyma are just another specialized parenchyma.
as summerized from
Esau, Katherine. Anatomy of Seed Plants. 2nd ed. 1977 John Wiley
& Sons, Inc
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